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Participant

Join Date: Mar 2009
Location: Honduras
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### How to Calculate Heat Losses Through Building Walls

03/31/2009 11:46 AM

With rising electricity bills for air conditioning, savings in buildings are looked for in all areas. Please provide information on how can I work walls.

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Guru

Join Date: Jun 2006
Location: San Diego
Posts: 3087
#1

### Re: How can I calculate heat losses thru buildings wall

03/31/2009 12:49 PM

My preference is Audel's as the most approachable.

Heat loss and cooling "loss" (actually heat infiltration) are two sides of the same coin, with slightly different individual problems.

Both share one common, unplanned air exchange between inside and outside is #1 enemy. So gaps, cracks, and leaks are the #1 source. Buy caulk.

#2 is insulating properties of materials. Slightly different for cooling, but well explained in any HVAC book.

While any discussion of heat retention focuses on wrapping insulating material around the space, heat rejection may do better with blocking radiant heat intrusion. So frequently one finds radiant barriers used where there is room, they need some space to be effective.

Then start looking at solar mass from a backward point of view. In the winter we try to get solar mass heated by letting sun fall on some slab of concrete. In the summer, you do not want that concrete heated; whether it is through a window in your house or directly outside your house.

So then one has to look at the color of the house, the color of the roof, plantings in the yard.

So there are many aspects, depending on year-round needs, that can be addressed in the structure design, the execution of the design, and the environment in which the design is placed.

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Join Date: Jul 2008
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#2

### Re: How to Calculate Heat Losses Through Building Walls

04/01/2009 1:30 AM

Heat losses through a wall can be calculated as follows:

Q = UA(Ti -To)

where Q is heat transferred through the wall, A is the area of the wall, Ti is the temperature in the room and To is the temperature outside; and finally U = 1/(1/hi + 1/kw + 1/ho) where hi is the convective heat transfer inside the room, kw is the average thermal conductivity of the wall, and ho is the convective heat transfer of the outside.

The hi and ho may be calculated as characteristic of natural convection. The kw, however, may be the most difficult to calculate because of the composite nature of the walls of buildings, but I would calculate it weighted this way: 1/tw(∑ kjtj) ; where tw is the wall thickness, tj is the thickness of the jth layer of the layers making up the wall, and kj is the thermal conductivity of the jth layer; and ∑ signifies summation over j from j=1 through j=n, and n is the number of layers of materials used in the wall.

I hope that this is helpful

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Guru

Join Date: Jun 2006
Location: San Diego
Posts: 3087
#3

### Re: How to Calculate Heat Losses Through Building Walls

04/01/2009 8:54 AM

I can always count on someone smarter coming along!

GA!

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Associate

Join Date: Jan 2009
Location: Houston, TX
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#4

### Re: How to Calculate Heat Losses Through Building Walls

04/02/2009 10:48 AM

I couldn't have said it better myself. This is a simple two-dimensional heat conduction problem, using Fourier's Law.